1. Field of the Invention
The present invention relates, generally to a method of controlling a dual clutch transmission and, more specifically, to a method for controlling the timing of the gearshift events of the dual clutch transmission by determining the optimum shift points based on vehicle acceleration or load.
2. Description of the Related Art
Generally speaking, land vehicles require a powertrain consisting of three basic components. These components include a power plant (such as an internal combustion engine), a power transmission, and wheels. The power transmission component is typically referred to simply as the “transmission.” Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle. Presently, there are two typical transmissions widely available for use in conventional motor vehicles. The first and oldest type is the manually operated transmission. These transmissions include a foot-operated start-up or launch clutch that engages and disengages the driveline with the power plant and a gearshift lever to selectively change the gear ratios within the transmission. When driving a vehicle having a manual transmission, the driver must coordinate the operation of the clutch pedal, the gearshift lever, and the accelerator pedal to achieve a smooth and efficient shift from one gear to the next. The structure of a manual transmission is simple and robust and provides good fuel economy by having a direct power connection from the engine to the final drive wheels of the vehicle. Additionally, since the operator is given complete control over the timing of the shifts, the operator is able to dynamically adjust the shifting process so that the vehicle can be driven most efficiently. One disadvantage of the manual transmission is that there is an interruption in the drive connection during gear shifting. This results in losses in efficiency. In addition, there is a great deal of physical interaction required on the part of the operator to shift gears in a vehicle that employs a manual transmission.
The second and newer choice for the transmission of power in a conventional motor vehicle is an automatic transmission. Automatic transmissions offer ease of operation. The driver of a vehicle having an automatic transmission is not required to use both hands, one for the steering wheel and one for the gearshift, and both feet, one for the clutch and one for the accelerator and brake pedal in order to safely operate the vehicle. In addition, an automatic transmission provides greater convenience in stop and go situations, because the driver is not concerned about continuously shifting gears to adjust to the ever-changing speed of traffic. Although conventional automatic transmissions avoid an interruption in the drive connection during gear shifting, they suffer from the disadvantage of reduced efficiency because of the need for hydrokinetic devices, such as torque converters, interposed between the output of the engine and the input of the transmission for transferring kinetic energy therebetween. In addition, automatic transmissions are typically more mechanically complex and therefore more expensive than manual transmissions.
For example, torque converters typically include impeller assemblies that are operatively connected for rotation with the torque input from an internal combustion engine, a turbine assembly that is fluidly connected in driven relationship with the impeller assembly and a stator or reactor assembly. These assemblies together form a substantially toroidal flow passage for kinetic fluid in the torque converter. Each assembly includes a plurality of blades or vanes that act to convert mechanical energy to hydrokinetic energy and back to mechanical energy. The stator assembly of a conventional torque converter is locked against rotation in one direction but is free to spin about an axis in the direction of rotation of the impeller assembly and turbine assembly. When the stator assembly is locked against rotation, the torque is multiplied by the torque converter. During torque multiplication, the output torque is greater than the input torque for the torque converter. However, when there is no torque multiplication, the torque converter becomes a fluid coupling. Fluid couplings have inherent slip. Torque converter slip exists when the speed ratio is less than 1.0 (RPM input>than RPM output of the torque converter). The inherent slip reduces the efficiency of the torque converter.
While torque converters provide a smooth coupling between the engine and the transmission, the slippage of the torque converter results in a parasitic loss, thereby decreasing the efficiency of the entire powertrain. Further, the torque converter itself requires pressurized hydraulic fluid in addition to any pressurized fluid requirements for the actuation of the gear shifting operations. This means that an automatic transmission must have a large capacity pump to provide the necessary hydraulic pressure for both converter engagement and shift changes. The power required to drive the pump and pressurize the fluid introduces additional parasitic losses of efficiency in the automatic transmission.
In an ongoing attempt to provide a vehicle transmission that has the advantages of both types of transmissions with fewer of the drawbacks, combinations of the traditional “manual” and “automatic” transmissions have evolved. Most recently, “automated” variants of conventional manual transmissions have been developed which shift automatically without any input from the vehicle operator. Such automated manual transmissions typically include a plurality of power-operated actuators that are controlled by a transmission controller or some type of electronic control unit (ECU) to automatically shift synchronized clutches that control the engagement of meshed gear wheels traditionally found in manual transmissions. The design variants have included either electrically or hydraulically powered actuators to affect the gear changes. However, even with the inherent improvements of these newer automated transmissions, they still have the disadvantage of a power interruption in the drive connection between the input shaft and the output shaft during sequential gear shifting. Power interrupted shifting results in a harsh shift feel that is generally considered to be unacceptable when compared to smooth shift feel associated with most conventional automatic transmissions.
To overcome this problem, other automated manual type transmissions have been developed that can be power-shifted to permit gearshifts to be made under load. Examples of such power-shifted automated manual transmissions are shown in U.S. Pat. No. 5,711,409 issued on Jan. 27, 1998 to Murata for a Twin-Clutch Type Transmission, and U.S. Pat. No. 5,966,989 issued on Apr. 04, 2000 to Reed, Jr. et al for an Electro-mechanical Automatic Transmission having Dual Input Shafts. These particular types of automated manual transmissions have two clutches and are generally referred to simply as dual, or twin, clutch transmissions. The dual clutch structure is most often coaxially and cooperatively configured so as to derive power input from a single engine flywheel arrangement. However, some designs have a dual clutch assembly that is coaxial but with the clutches located on opposite sides of the transmissions body and having different input sources. Regardless, the layout is the equivalent of having two transmissions in one housing, namely one power transmission assembly on each of two input shafts concomitantly driving one output shaft. Each transmission can be shifted and clutched independently. In this manner, uninterrupted power upshifting and downshifting between gears, along with the high mechanical efficiency of a manual transmission is available in an automatic transmission form. Thus, significant increases in fuel economy and vehicle performance may be achieved through the effective use of certain automated manual transmissions.
The dual clutch transmission structure may include two dry disc clutches each with their own clutch actuator to control the engagement and disengagement of the two-clutch discs independently. While the clutch actuators may be of the electromechanical type, since a lubrication system within the transmission requires a pump, some dual clutch transmissions utilize hydraulic shifting and clutch control. These pumps are most often gerotor types, and are much smaller than those used in conventional automatic transmissions because they typically do not have to supply a torque converter. Thus, any parasitic losses are kept small. Shifts are accomplished by engaging the desired gear prior to a shift event and subsequently engaging the corresponding clutch. With two clutches and two inputs shafts, at certain times, the dual clutch transmission may be in two different gear ratios at once, but only one clutch will be engaged and transmitting power at any given moment. To shift to the next higher gear, first the desired gears on the input shaft of the non-driven clutch assembly are engaged, then the driven clutch is released, and the non-driven clutch is engaged.
This requires that the dual clutch transmission be configured to have the forward gear ratios alternatingly arranged on their respective input shafts. In other words, to perform up-shifts from first to second gear, the first and second gears must be on different input shafts. Therefore, the odd gears will be associated with one input shaft and the even gears will be associated with the other input shaft. In view of this convention, the input shafts are generally referred to as the odd and even shafts. Typically, the input shafts transfer the applied torque to a single counter shaft, which includes mating gears to the input shaft gears. The mating gears of the counter shaft are in constant mesh with the gears on the input shafts. The counter shaft also includes an output gear that is meshingly engaged to a gear on the output shaft. Thus, the input torque from the engine is transferred from one of the clutches to an input shaft, through a gear set to the counter shaft and from the counter shaft to the output shaft.
Gear engagement in a dual clutch transmission is similar to that in a conventional manual transmission. One of the gears in each of the gear sets is disposed on its respective shaft in such a manner so that it can freewheel about the shaft. A synchronizer is also disposed on the shaft next to the freewheeling gear so that the synchronizer can selectively engage the gear to the shaft. To automate the transmission, the mechanical selection of each of the gear sets is typically performed by some type of actuator that moves the synchronizers. A reverse gear set includes a gear on one of the input shafts, a gear on the counter shaft, and an intermediate gear mounted on a separate counter shaft meshingly disposed between the two so that reverse movement of the output shaft may be achieved.
While these power-shift dual clutch transmissions overcome several drawbacks associated with conventional transmissions and the newer automated manual transmissions, it has been found that controlling and regulating the automatically actuated dual clutch transmissions is a complicated matter and that the desired vehicle occupant comfort goals have not been achievable in the past. There are a large number of events to properly time and execute within the transmission for each shift to occur smoothly and efficiently. Conventional control schemes and methods have generally failed to provide this capability. Accordingly, there exists a need in the related art for better methods of controlling the operation of dual clutch transmissions.
One particular area of control improvement that is needed is in the timing of the events in the power-shifting of the dual clutch transmission. As discussed above, power shifting is actually the automatic gear shifting process of the dual clutch transmission. The nature of the dual clutch transmission, that is, the manual style configuration discussed above that employs automatically actuated disc type clutches, requires accurate control of the clutch engagement and thus the torque transferred across them during the gear shifting process. Additionally, the movement of the synchronizers requires accurate control in the gear-shifting event. It is desirable to operate the synchronizers and the clutches of the dual clutch transmission so that the automatic gear shifting process is smoothly and efficiently controlled by varying the amount of torque transferred across each clutch as the clutch torque driving the off-going gear is minimized and the clutch torque driving the on-coming clutch is maximized. The efficiency and smooth operation of the shifting event is directly related to the control and timing of each portion of the shift. More specifically, it is critical to control the timing of the shift in relation to the demand for vehicle acceleration and for variations in vehicle load caused by road and driving conditions.
Current control methods have the general capability to operate the clutches and synchronizers as needed. However, the prior art dual transmission clutch control schemes are incapable of adequately providing for the fine timing and control of the shift event necessary to satisfy this need. Specifically, they lack the ability to accurately control when the shift occurs to achieve the high degree of accuracy needed for peak fuel efficiency and driveability in shifting between the gears of the transmission under all vehicle loading conditions and throttle demands. For example, when rapid acceleration is called for, with low or light load conditions on the vehicle, engine inefficiency and even damage may occur if the shift timing is not adjusted to occur sooner as a means for compensating for the rapid acceleration when compared to the shift timing of a more leisurely acceleration. In other words, if the timing of the shift is not adjusted to account for the rapid increase in engine speed such that the shift event is held to a slower shifting pace, the engine will likely overspeed. Likewise, if rapid acceleration is called for but the load on the vehicle is heavier, such as when the vehicle encounters a steep road incline, the resultant vehicle acceleration will be slower. In this case, the shift should occur later to coincide increasing load to prevent a premature gearshift. Downshifting events also require timing control depending on the deceleration demands and vehicle loading to provide smooth and efficient control of the transmission and engine. Current control methods for shifting a dual clutch transmission generally concern themselves with simple engagement and disengagement of the clutch assemblies and synchronizers and fail to adequately provide for the corresponding timing control of all aspects of the shift event including engine speed control during the shift and the differences in upshifting and downshifting.
In that regard, some prior control methods for the gear shifting of dual clutch transmissions have attempted to overcome these inadequacies by using a control algorithm. For example, one known method provides an algorithm to control the movement of electrical clutch actuators, and thus the engagement of the clutches, to prevent torque interruption during upshifts of a dual clutch transmission. While the application of this particular algorithm is functionally adequate for its intended use, it still has certain drawbacks that leave room for improvement.
Particularly, while this and other known dual clutch transmission shifting approaches attempt to provide a power-shift in which there is no interruption of torque transfer, none of the current methods provides for variable shift points and variable engine speed profiles for smooth and efficient torque transfer under varying conditions. Additionally, certain prior art methods utilize an engine performance map that attempts to predict expected engine output torque and sets the clutch position and shift points based on those predictions so that this control method is reactive to predicted engine output. The drawback of this control approach is that the values of the considered variables can fluctuate greatly and a stored map of predictions cannot be adequately relied upon to predict the actual engine output. Accordingly, there remains a need in the art for a method to operatively and actively control the timing of the gearshifts in a dual clutch transmission by varying the shift points so that both upshifts and downshifts are efficiently and smoothly performed and optimum engine efficiency and safe operation is maintained.